The invention relates to a method of transmitting signals between an adaptive antenna of a base station and a mobile user equipment in a telecommunication network.
In an Universal Mobile Telecommunication System (UMTS), a base station communicates with one or more mobile user equipments. The base station comprises an adaptive antenna which sends data to the user equipment in a so-called downlink and receives data from the user equipment in a so-called uplink.
The adaptive antenna comprises two or more antenna elements.
In the uplink from a specific mobile user equipment, the base station receives several signals from the adaptive antenna. Each one of the signals is weighted with a complex factor. Then, the weighted signals are accumulated in order to create an uplink signal which is further processed within the base station. As an example, the complex factor may relate to the amplitude and the phase of the respective received signal.
The complex factors are determined by a so-called uplink adaptive antenna function. The complex factors are calculated such that an optimal uplink signal is created. The optimum of the uplink signal may be seen in the sense of an optimal signal-to-interference ratio of the created uplink signal. However, also other criteria for an optimal uplink signal may be selected.
The uplink adaptive antenna function has the further task to keep the complex factors optimal over the duration of a communication. For that purpose, the complex factors are calculated e.g. a few times per second.
The calculation of the complex factors can be carried out by the base station without any feedback or the like. A number of algorithms are known in the prior art to calculate the complex factors. As well, algorithms are known to determine an approximate direction of the specific mobile user equipment in respect to the base station based on the calculated complex factors.
For example, a closed loop power control algorithm may be used in the uplink to adjust the signal-to-interference ratio of the signal received from a specific mobile user equipment to a given target value.
In the downlink to a specific mobile user equipment, the base station creates a downlink signal which is forwarded to the two or more antenna elements of the adaptive antenna. Each one of the two or more antenna elements weights the downlink signal with a complex factor. Then, the weighted signals are sent out by the two or more antenna elements of the adaptive antenna. Again, the complex factor may relate to the amplitude and the phase of the respective signal.
The weighting of the signals sent out by the two or more antenna elements of the adaptive antenna allows to adjust the beam of the adaptive antenna into the direction of the specific mobile user equipment.
The complex factors are determined by a so-called downlink adaptive antenna function.
The complex factors are calculated such that signals sent out by the two or more antenna elements of the adaptive antenna are optimal. The optimum of these signals may be seen in the sense of an optimal signal-to-interference ratio of the signal received by the mobile user equipment. However, also other criteria for an optimal downlink signal may be selected.
The downlink adaptive antenna function has the further task to keep the complex factors optimal over the duration of a communication. For that purpose, the complex factors are calculated e.g. a few times per second.
The calculation of the complex factors is carried out by the base station. However, in the case of a dependency of the complex factors from the signal-to-interference ratio of the signal received by the mobile user equipment, some kind of feedback from the mobile user equipment is necessary. A possible algorithm to calculate the complex factors is described in EP 1 067 710 A1.
A problem occurs in the downlink when a new mobile user equipment is turned on. In this situation, the base station does not know the location of the new mobile user equipment so that an adjustment of the beam of the adaptive antenna into the direction of the new user equipment is not possible.
This situation may be solved as follows:
It is possible to “start” the adjustment of the beam of the adaptive antenna with a very broad range of the used complex factors. Then, these complex factors may be optimized according to EP 1 067 710 A1. However, this procedure has the disadvantage that the quality of the downlink signal received in the new mobile user equipment during the “start” of the adjustment might be low.
If the adaptive antenna for the uplink and the adaptive antenna for the downlink are identical, then it is possible to use the fact that the direction of the new user equipment in respect to the base station is known from the first transmission in the uplink. As described, the base station calculates complex factors of the uplink signal received by the adaptive antenna which, among others, may correspond to the direction of the specific user equipment in respect to the base station. These complex factors of the uplink may then be used to derive those complex factors of the downlink which also correspond to the direction of the new user equipment.
If the uplink and the downlink use the identical carrier frequency, then, the complex factors calculated from the uplink may be identically used for the downlink.
However, as it is the case for UMTS, if the uplink and the downlink use different carrier frequencies, then the complex factors of the uplink can only be used for the downlink if the signal paths of the uplink and the downlink are calibrated.
However, the calibration of signal paths is very complex and expensive and has to be carried out regularly e.g. due to temperature changes or aging.
Another problem occurs in the uplink. As described, the complex factors are determined by the uplink adaptive antenna function such that an optimal uplink signal is created. However, it is possible that the uplink adaptive antenna function does not—for whatever reason—detect the absolute optimum but only a local optimum. As a result, the created uplink signal is not optimal.